This invention relates generally to refrigerant expansion control in refrigeration systems and more particularly concerns electronic circuitry for controlling pulse width modulated solenoid valves. As used herein, the term "refrigeration system" refers to refrigerators, air conditioners or any other system which produces a refrigeration effect.
Conventional refrigeration systems used in household refrigerators typically operate on the simple vapor compression cycle. Such a cycle includes a compressor, a condenser, an expansion device, and an evaporator all connected in series in the order given and charged with a refrigerant. The refrigerant is compressed by the compressor to high temperature and pressure and then condensed by the condenser where it loses heat to the ambient. The liquid refrigerant next flows through an expansion device, such as an expansion valve or a capillary tube, so that it undergoes adiabatic expansion. The now low pressure refrigerant flows through the evaporator and is vaporized by absorbing heat from air passing over the evaporator. The cooled air is used to refrigerate one or more refrigerator compartments. The gaseous or mostly gaseous refrigerant exiting the evaporator is returned to the compressor via a suction line to repeat the cycle.
Household refrigerators typically use a capillary tube to control refrigerant expansion because it is a simple, low cost device. However, capillary tubes have a number of limitations as expansion devices. For instance, capillary tubes must be made very long to allow an inside diameter which is manufacturable and large enough to avoid clogging. This needed length takes up space in the refrigerator. The use of capillary tube expansion control also requires very precise refrigerant charging operations during production because the flow rate through the capillary tube is highly sensitive to the amount of refrigerant charge in the system.
Furthermore, a capillary tube can be sized to provide the optimum refrigerant flow rate for only one operating condition. Capillary tubes are thus typically sized to provide the optimum flow rate for normal operating conditions. This means that when the refrigeration cycle begins (as well as under high load conditions), the capillary tube is undersized, and the evaporator is starved of refrigerant. This reduces the cooling capacity and efficiency of the refrigerator. Near the end of the refrigeration cycle, the capillary tube will be oversized and the evaporator will be flooded, again reducing efficiency. Because of this, cycle efficiency using capillary tube expansion is considerably below that attainable with active expansion control.
However, active expansion control, in the form of conventional thermostatic expansion valves, does not work well in household refrigerators. While thermostatic expansion valves are often used in automotive air conditioning and commercial refrigeration systems which have large refrigerant flow rates, they cannot be made with orifices small enough to regulate the very low flow rates (typically 10-12 lb/hr) of household refrigerators. That is, to achieve the required pressure drop, the valve orifice would need to be on the order of 10 mils or less, a size that is impractical to manufacture and very susceptible to plugging. A pulse width modulated solenoid valve is a possible alternative to thermostatic expansion valves, but such a valve requires a controller which can respond to changes in an operating condition of the refrigeration system.
Accordingly, there is a need for a control circuit which can control a pulse width modulated solenoid valve so as to produce maximum cooling capacity and efficiency in a refrigeration system.